Heat acclimation, evidenced by expanded plasma volume, higher sweat rates, lower heart rates, and a lower core temperature during exercise in a hot environment, has been achieved traditionally through the use of 8 to 12 days of walking in dry heat (40 C; 24 C WB) at 5.6 km/hr (40% V02 max) for 60 to 85 min/day. An acclimation protocol using shorter duration, higher intensity exercise may more accurately represent the thermal and exercise stresses commonly experienced by individuals exposed to heat during recreational and occupational activities. Project A. The goal of this proposed research is to determine if the usual benefits associated with heat acclimation can be achieved using a shorter, more intense protocol. The findings of this study will provide information to help those exercising in the heat to avoid heat related injuries, and will assist in the prescription of exercise programs of individuals preparing for strenuous activities in a hot environment. Ten untrained healthy men will undergo a heat- exercise test (HET) consisting of a 90 min treadmill walk at 5.6 km/hr with a 6% grade in the heat (40 C, 23.5% RH), before, midway, and after 8 days of heat acclimation using either the traditional protocol described above, or the experimental protocol (treadmill running 30 min/day at 70% of V02max). To more closely investigate the physiological responses associated with acclimation, hematocrit, hemoglobin, total protein, cortisol, catecholamines, creatine kinase, renin, aldosterone, sodium, and potassium will be measured before and during the HET. Resting plasma volume, and heart rate, mean body and rectal temperatures, and sweat electrolytes during the HET will be measured to determine the degree of heat acclimation. Project B. It has been observed in this laboratory that hypertensive subjects are less tolerant of heat, a characteristic that may be related to an inability to fully expand their plasma volumes during acclimation. Conversely, a fully expanded plasma volume may worsen existing volume-related problems and impose an additional cardiovascular stress. The goal of this proposed research is to fully examine the cardiovascular and hormonal responses of hypertensive subjects during heat acclimation. The findings of this study will elucidate some of the physiological responses of the hypertensive patient during exercise in the heat, and will provide information that may help reduce the risks associated with heat exposure for a population who already run a high risk because of their disease. Ten non-medicated, middle aged hypertensive (140-160 systolic, 95-110 diastolic) males free of other primary risk factors associated with ischemic heart disease, and ten normotensive males will undergo a HET before, midway, and after 8 days of acclimation using the traditional protocol described above. The same physiological and blood constituent measurements made in Project A will be made in this project to assess general responses to heat and those directly associated with heat acclimation. These responses will be compared to those of the normotensive controls.